1. Field of the Invention
The present invention is directed to a process for making a graft propylene copolymer. More particularly, the present invention is directed to an improved process for copolymerizing propylene polymers by the free-radical grafting thereon of a monomer under molten conditions.
A blend of a polyolefin and a second thermoplastic resin permits the extension of one or more uses particularly suited to the polyolefin or the second resin to the other resin to which use that other resin is unsuited. For example, polypropylene combined with nylon provides superior high temperature performance and, at the same time, good low moisture absorption characteristics. Alone a polypropylene homopolymer has poor high temperature performance. Similarly, nylon polymers are characterized by poor low moisture absorption.
A major barrier to combining a polyolefin with a second non-polyolefinic thermoplastic resin in the prior art has been the absence of compatibility of the two resins. Those skilled in the art are aware that polyolefins, being non-polar, are not compatible with other more polar thermoplastic resins. Obviously, a compatibilizing agent, which is itself compatible with the two resins, and thus permits the blending of such resins, would enhance increased utilization of the two thermoplastic resins which constitute the blend or alloy by extending the field of usefulness of those individual thermoplastics.
It is well known that a polymerizable, ethylenically unsaturated carboxylic acid or acid derivative, or as it is better known because of the common use of maleic anhydride, a maleated polyolefin, serves as an effective compatibilizer. Unfortunately, the high cost of maleated olefin copolymer compatibilizers has limited commercial development of the above discussed thermoplastic blends. Thus, there is a continuing need to develop better and more cost effective methods of preparing maleated polyolefins in order to fully exploit the potential of thermoplastic alloys and blends.
A particularly important class of maleated olefin copolymer compatibilizers are maleated polypropylene resins. The importance of this class of compatibilizers lies in the ability of this class of compatibilizers to permit the blending of polypropylene with other thermoplastics. Such compositions find particular utility in laminate and adhesive applications.
To exploit these burgeoning laminate and adhesive markets new processes have been developed to produce maleated polypropylenes. These processes involve graft copolymerization onto polypropylene of maleating agents. There are three types of graft copolymerization processes that successfully produce the desired maleated polypropylene or, more generally, ethylenically unsaturated carboxylic acid or acid derivative-propylene graft copolymers discussed above. These processes are melt graft copolymerization, solution graft copolymerization and solid state graft copolymerization. Each of these processes have their advantages and disadvantages. However, melt polymerization combines the advantages of continuous polymerization potential, which it shares with solution polymerization, and the absence of safety and health hazards, a characteristic which inhibits the use of solution polymerization processing. It is noted that although solid state copolymerization is, like melt polymerization, free of safety and health hazards, this process suffers from the inability to be operated continuously.
A particular problem associated with the graft copolymerization of polypropylene and an ethylenically unsaturated carboxylic acid or acid derivative is the necessity of including a free radical generating agent in the copolymerization reaction. Whereas free radical generating agents, such as organic peroxides, tend to effect crosslinkage of other polyolefins, they have a visbreaking effect on polypropylene. That is, the exposure of polypropylene to a free radical generating agent, opposite to other polyolefins, results in a decrease of polypropylene molecular weight. Thus, the maleation of polypropylene, that is, the copolymerization of an ethylenically unsaturated carboxylic acid or acid derivative and propylene polymer, is the most difficult of all polyolefin graft copolymerizations to effect.
2. Background of the Prior Art
There is much prior art directed to the grafting of polymerizable ethylenically unsaturated carboxylic acids or acid derivatives onto propylene polymers. The references discussed below are considered to be particularly relevant to this graft copolymerization process.
Lambla et al., Multiphase Polymers: Blends and Ionomers, ACS Symposium Series 395, Chap. 3, 76-79 (1989) describes a study directed to processes for producing graft copolymers of maleic anhydride and polypropylene. The results of this study establish that a process in which polypropylene is melted, in the presence of dicumyl peroxide acting as an initiator, with a stoichiometric amount of maleic anhydride and styrene produces the highest degree of grafting of the maleic anhydride onto the polypropylene polymer compared to polymerization of maleic anhydride in the absence of a coagent or maleic anhydride in the presence of the coagent butyl acrylate.
U.S. Pat. No. 5,001,197 relates to a process of maleating a polyolefin, preferably polypropylene, in which the polyolefin, especially polypropylene, molecular weight is not significantly degraded during the maleation polymerization reaction. This maleation reaction is accomplished by the use of a class of peroxide initiators which decompose to produce radical fragments having the structural formulae --R or --OR, where R is an alkyl group. Two particularly preferred peroxides within this class of initiators are t-butyl peroxypivalate and isononanoyl peroxide.
U.S. Pat. No. 5,079,302 describes a solid state free radical induced polymerization reaction involving a polymerizable polymer, usually polypropylene, which is reacted with a free radical polymerizable monomer, preferably maleic anhydride, in the presence of an allylic cyanurate or isocyanurate. This reaction occurs in the solid state, that is, at a temperature below the melting temperature of the polymeric, i.e. polypropylene, reactant.
U.S. Pat. No. 5,137,975 sets forth a method for melt grafting polypropylene with an .alpha.,.beta.-unsaturated carboxylic acid or anhydride in the presence of a minor amount of an aqueous solution of an alkaline material. No coagent is utilized in the graft copolymerization reaction described in the '975 patent.
Although the processes of the above discussed references provide new methods of grafting polymerizable ethylenically unsaturated carboxylic acids or acid derivatives onto a propylene polymer they still do not provide a process which produces an effective graft propylene copolymer, i.e. a copolymer having an effectively high carboxylic acid carboxylic acid or acid derivative concentration, in commercially acceptable quantities.